Jpn J Radiol (2011) 29:413–422
DOI 10.1007/s11604-011-0572-z

ORIGINAL ARTICLE

Comparison of diagnostic and prognostic capabilities of 18F-FDG-PET/
CT, 131I-scintigraphy, and diffusion-weighted magnetic resonance imaging
for postoperative thyroid cancer
Shigeki Nagamachi · Hideyuki Wakamatsu
Shogo Kiyohara · Ryuichi Nishii · Youichi Mizutani
Seigo Fujita · Shigemi Futami · Hideo Arita
Masaomi Kuroki · Hiroshi Nakada · Noriko Uchino
Shozo Tamura · Keiichi Kawai

Received: September 19, 2010 / Accepted: February 3, 2011
© Japan Radiological Society 2011

Abstract
Purpose. The first aim of this study was to compare thedetectability of metastasis of postoperative differentiated thyroid cancer (DTC) among 131I whole body scintigraphy (IWBS), fluorodeoxyglucose-positron emission
tomography/computed tomography (FDG-PET/CT),and diffusion-weighted magnetic resonance imaging(DWI). The second aim was to clarify the association
between the image pattern and prognosis.
Materials and methods. We evaluated 70 postoperative
DTC patients on both a patient basis and an organ basis(lymph nodes, lung, bone), and we analyzed the correlation between the image pattern and the prognosis.
Results. For the patient-basis analysis, the detectability
by IWBS, PET/CT, and DWI was 67.1%, 84.2%, and57.6%, respectively. IWBS provided complementary
information to that provided by PET/CT in 11 of 70(15.7%) cases. For the organ-basis analysis, IWBS was
the best detector for lymph node metastasis (72.4%).
PET/CT was superior to IWBS for detecting metastasis

S. Nagamachi (*) · H. Wakamatsu · S. Kiyohara · R. Nishii ·Y. Mizutani · S. Fujita · S. Futami · H. Arita · M. Kuroki ·H. Nakada · N. Uchino · S. Tamura
Department of Radiology, School of Medicine, Miyazaki
University, 5200 Kihara, Kiyotake-cho, Miyazaki 889-1692,
Japan
Tel. +81-985-85-1510 (ext. 2244); Fax +81-985-85-7172
e-mail: snagama@med.miyazaki-u.ac.jp
K. Kawai
Faculty of Health Science, School of Medicine, Kanazawa
University, Kanazawa, Japanof bone (85.7% vs. 71.4%) and lung (94.1% vs. 62.7%).
For the correlation analysis, PET and DWI positivity
were the factors predicting a poor prognosis.
Conclusion. PET/CT was the best modality for detecting
metastases in postoperative DTC patients, although
IWBS provided complementary information. Because
PET/CT and DWI gave similar information (e.g., positivity) suggesting poor prognoses, the combination of
IWBS and DWI might be the method of choice for monitoring postoperative DTC.
Key words 131I whole-body scintigraphy · DWI · DTC ·F-FDG-PET/CT · Prognosis

18

Introduction
18

F-fluorodeoxyglucose positron emission tomography(FDG-PET) is a rapidly evolving imaging modality thathas gained widespread acceptance in oncology.1,2 Clinical experience with FDG-PET in patients with differentiated thyroid cancer (DTC) has been reported.3–8 Several
investigators have reported that FDG-PET and 131I
whole-body scanning (WBS) had complementary rolesin the detection of recurrent or metastatic DTC.3,4,9,10
Well-differentiated thyroid cancer was positive for 131I
uptake and negative for FDG uptake, whereas poorly
differentiated thyroid cancer was negative for 131I uptake
and positive for FDG uptake.3–5,10 The combined imagesof 131I WBS and FDG-PET in the follow-up of DTC had
a sensitivity of approximately 95% for detecting recurrence and metastasis.5

414

Few studies have compared FDG-PET/CT with 131I
whole-body scintigraphy (IWBS).7,11,12 In particular, few
studies have compared FDG-PET/CT with diffusionweighted MRI imaging (DWI), which is known to be
useful for diagnosing various cancers based on imaging
the molecular mobility of water.13–20 Although DWI is
not a routine investigation when evaluating thyroid
cancer, it may become one of the choices owing to theversatility of MRI for tumor diagnosis.
To the best of our knowledge, the current study is thefirst to compare directly the findings of FDG-PET/CT
and DWI in patients with DTC after total thyroidectomy. In addition, few studies have been conducted on
the correlation of image patterns and prognosis.21,22
The aim of this study was to evaluate the clinicalsignificance of whole-body FDG-PET/CT in DTC
patients by comparing the results with those obtainedwith IWBS and DWI. We also investigated the correlation of the prognosis with the image pattern.

Methods
Patients
This study included 70 patients with known metastaticdifferentiated thyroid cancer after total thyroidectomy(22 men, 48 women; ages 27–72 years, mean ± SD 55.2
± 23 years). They were scheduled for 131I therapy between
July 2005 and June 2009. There were 62 patients withpapillary cancer and 8 with follicular cancer. All of themhad stopped taking thyroxine (T4) for 3 weeks for 131I
therapy. The thyroid-stimulating hormone (TSH) level
was >100 μIU/ml for all patients at the time of 131I oral
ingestion. Metastasis or recurrence had been diagnosed
on the basis of the increased thyroglobulin levels (>30 ng/ml), positive cytology findings, or positive findings onfollow-up imaging including FDG-PET/CT, 131I scintigraphy, DWI magnetic resonance imaging (MRI), computed tomography (CT), ultrasonography (US), andbone scintigraphy. Written consent was obtained fromthe patients for all of the imaging studies.
Imaging
We performed FDG-PET/CT using a whole-body PET/CT system (Biograph 16; Siemens/CTI, Knoxville, TN,
USA). Patients fasted for at least 5 h. Whole-body
FDG-PET/CT images were obtained 60 min after injection of 185 MBq FDG using the three-dimensional
method. The blood glucose level, measured just before
tracer administration, was <110 mg/dl in all patients. All
patients were asked to remain at rest and quiet justJpn J Radiol (2011) 29:413–422

before scanning. The images were obtained from the topof the brain to the femur in all patients. We did not use
intravenous contrast media. All PET images were reconstructed using iterative algorithms (Fourier rebinningplus attenuation-weighted ordered-subset expectation
maximization, two iterations, eight subsets, 5-mm
Gaussian filter) with CT-based attenuation correction.
The data were reconstructed with a 256 × 256 matrix and
3 mm slice thickness. To detect metastases in the lungfield by low-dose CT, we reconstructed transaxial images,
visualized using the window level, −600 Hounsfield units(HU) and width 1600 HU. We conducted all FDG-PET/CT examinations under conditions without the influenceof the TSH increase, before thyroid hormone depletion.
To avoid possible metabolic stunning,23 all FDG-PET/
CT studies were performed 3 weeks before 131I therapy.
131
I whole-body scintigraphy was performed using adual-head gamma camera (eCAM; Siemens/CTI) withhigh-energy collimators. The images were acquired 3–4
days after oral ingestion of the therapeutic dose (3.7
GBq) of 131I.
The DWI was performed using a 1.5-T MRI imaging
system (1.5-T Excelart Vantage, ZGV; Toshiba, Tokyo,
Japan). Image analysis was done using the Power Plus
Package (Toshiba). Both neck and chest images wereobtained under the following conditions: TR/TE 11
000/70 ms; slice 6 mm; gap 1 mm; b value 1000 s/mm2;
number of excitations (NEX) 4; field of view (FOV)
240 mm; matrix 256 × 256; flip angle (FA) 90°. Although
the MRI protocol called for the imaging to be done 3–4
days before 131I administration and after hospitalization,
it could not be done in 11 cases owing to unexpected
machine problems or to the patient’s condition, such as
claustrophobia or hypothyroidism-induced depression
or anxiety neurosis.24
AnalysisTwo experienced nuclear medicine physicians visuallyinterpreted the FDG-PET/CT and IWBS images until
they arrived at consensus. Lesions were defined as “positive” when there was a definite localized area of higher
uptake than that in the surrounding normal tissue. In
the interpretation of IWBS at ablation 131I therapy, we
excluded uptake in the neck and upper mediastinum,
so-called thyroid bed uptake, from the evaluation. With
respect to equivocal uptake lesions, agreement wasreached on the basis of the consensus of two nuclear
medicine physicians. Positive findings were confirmed by
the presence of carcinoma on histological examination
or were confirmed by follow-up imaging examination,
including US or diagnostic CT in the presence of persistent abnormal or increasing thyroglobulin (Tg) levels.

Jpn J Radiol (2011) 29:413–422

Similarly, all DWI images were visually interpreted bythree experienced radiologists including head and neckexpert(s) and a chest specialist. There was no exchange
of information between the nuclear medicine physicians
and the radiologists. For the interpretation of DWI
images, “positive” was diagnosed when a definite abnormal localized are of diffusion was noted as a highersignal lesion in accord with tumor location confirmed byhistological examination or confirmed by follow-up
imaging examination. With respect to equivocal lesions,the final decision was done by agreement of three
radiologists.
The analyses of obtained images were done on botha patient basis and an organ basis. For the patient-basis
analysis, “positive” was defined as a classification of“positive” in any organ. Organ-basis analysis was conducted by dividing anatomical regions into three organs:cervical or mediastinal lymph nodes; lung; bone. A classification of “positive” was assigned when at least one
metastasis was detected in the organ (lungs, bones,
lymph nodes). Multiple lesions in one organ wereregarded as one lesion. Image patterns obtained with thethree imaging methods were compared visually withregard to the distribution of metastasis.
We compared the possible influencing factors for
influencing the prognosis in two groups: good or poor.
Age, sex, serum Tg level before the examination, numberof times therapy was applied, and histological differentiation were also possible factors. Based on (1) changes
in the image findings and/or (2) changes in the serum Tg
level being excellent indexes for persistent disease andrecurrent disease diagnosis,25,26 we defined the “poor”
prognosis group by referencing either condition, (1) or(2), in the correlation analysis. Thus, (1) the value ofserum Tg persistently increased or re-increased during
the follow-up periods after the last 131I therapy; and/or(2) the lesion’s intensity or number increased duringfollow-up 1 year after the last 131I treatment. The patients
who did not exhibit either of these conditions were categorized as being in the “good” prognosis group. The
mean follow-up period from the last 131I therapy was
4.6 ± 0.6 years.
Statistical analysis
To determine the factors associated with survival rate,
we performed multivariate analysis using the hazard
model analysis of Cox. Factors included in analysis weresex, age, number of 131I treatments, image findings,
serum Tg level, and histological examination of surgical
specimens. Finally, we examined the association between
FDG or DWI positivity and the survival rate using
the Kaplan-Meier method. Continuous variables were415

expressed as a mean ± SD and were tested by Student’s
t-test. Noncontinuous variables were analyzed by thechi-squared test. Differences were considered significant
with P < 0.05.

Results
During the patient-basis analysis, 47 of 70 patients(67.1%) were detected by 131I-WBS and 34 of 59 (57.6%)
by DWI. By means of FDG-PET/CT, the detectability
notably improved to 84.2%. In all, 20 (28.6%) of 70
patients were detected only by FDG-PET CT or DWI(Table 1, Fig. 1a).
In the organ-basis analysis, IWBS was the best modality (72.4%) for detecting lymph node metastases (Table
2, Fig. 1b). The detectability of lymph node metastases
by other modalities was 61.7% with DWI and 67.2%
with PET/CT. Both IWBS and PET/CT showed concordance on 23 lesions (39.7%). Although findings of
DWI were similar to those with FDG-PET/CT, six
lesions (10.3%) were detected only by PET/CT.
As regards bone metastasis, PET/CT showed thehighest detectability rate (85.7%). The detectability with
IWBS (71.4%) was relatively lower than that with othermodalities. The FDG uptake accorded with DWI highsignal intensity in 13 of 17 lesions (76.5%) for which both
FDG-PET/CT and DWI were performed (Fig. 1c). For
the diagnosis of lung metastasis, FDG-PET/CT showed
Table 1. Detectability of metastasis (patient basis)
131

DWI

FDG-PET/CT

47/70
67.1%

34/59
57.6%

59/70
84.2%

IWBS

131
IWBS, iodine-131 whole-body scintigraphy; DWI, diffusionweighted magnetic resonance imaging; FDG-PET/CT, fluorodeoxyglucose-positronemission
tomography/computed
tomography

Table 2. Detectability of metastasis (organ basis)
Organ
Lymph node (n = 58)
131
IWBS
DWI (47a)
FDG-PET/CT
Lung (n = 51)
131
IWBS
DWI (40a)
FDG-PET/CT
Bone (n = 21)
131
IWBS
DWI (17a)
FDG-PET/CTaTotal number of DWI scans performed

No. detected
42 (72.4%)
29 (61.7%)
39 (67.2%)
32 (62.7%)
20 (50.0%)48 (94.1%)
15 (71.4%)
13 (76.5%)
18 (85.7%)

416
Fig. 1. Detectability ofmetastases illustrated by
the Venn diagram. a
Patient-based analysis (n= 70). DWI, diffusionweighted imaging;
FDG-PET/CT,
18
F-fluorodeoxyglucose
positron emission
tomography/computed
tomography; 131I,
iodine-131. b–d
Organ-based analyses. b
Lymph node (n = 58). c
Bone (n = 21). d Lung(n = 51)

Jpn J Radiol (2011) 29:413–422
Detectability of metastasis: patient basis (n=70)

Detectability of lymph node metastasis: organ basis (n=58)

DWI

DWI
0
20

0
10

0
14

3(2)
FDGPET/CT

0
19

6(4)

11(6)

22(3)

FDGPET/CT

131I

19(7)

4

131I
( ): DWI was not donea( ): DWI was not doneb

Detectability of bone metastasis: organ basis (n=21)

Detectability of lung metastasis: organ basis (n=51)

DWI

DWI

0

0

4
9
2
FDGPET/CT

3(2)

10

0

10
9(3)

3(2)

FDGPET/CT

131I

cexcellent detectability (94.1%) by visualizing small metastatic nodules with no FDG uptake (Table 2, Fig. 1d).
A comparison of the clinical factors, including tracer
uptake patterns, between the good prognosis and poor
prognosis groups is summarized in Table 3. Regarding
the imaging findings, both FDG and DWI positivity
were significant prognostic indicators. However, 131I
uptake pattern had no prognostic value. The histologicaldifferentiation, stage, and prior 131I therapy were alsosignificant determinants (Table 3). Patients in the poor
prognosis group were significantly older than those in
the good prognosis group. The Tg level is also significantly higher in the poor prognosis group. Multivariate
analysis indicated that age and the FDG positivity weresignificant factors influencing the survival rate (Table 4).
Univariate analysis using the Kaplan-Meier method
proved that the survival rate of the FDG-positive groupwas significantly lower than that of the FDG-negative
group. Similarly, the survival rate of the DWI-positive
group was significantly lower than that of the DWInegative group (Fig. 2). Representative FDG-negative
cases and FDG-positive cases are demonstrated in Figs.
3 and 4, and in Figs. 5 and 6, respectively.
Discussion
Our current results demonstrated that FDG-PET/
CT provided excellent detectability in the detection of( ): DWI was not done

d

19(6)

0
3(2)
131I

( ): DWI was not done

metastatic thyroid cancer on the patient-basis analysis,
which agrees with data obtained by FDG-PET/CT
that has recently become available.27–29 In the previous
report, FDG-PET was reported to play a complementary role for IWBS.2–5 However IWBS provided information complementary to that obtained with PET/CT(15.7%) in the current study. With regard to lung metastases, previous studies showed FDG-PET cannot adequately assess nodules <6 mm.4,30 Using FDG-PET/CT,
the detectability was impeccable for detecting small lungmetastatic nodules, including FDG-negative uptake
lesions. Nevertheless, the lack of resolution of low
radiation dose CT renders it insufficient for visualising
all miliary metastatic nodules. The problem should befurther analyzed using diagnostic high-dose radiationCT.
As regards bone metastases, FDG-PET/CT showed
excellent detectability. Nonetheless, it could not detect
the FDG-negative metastatic lesions. Possibly, FDGnegative and 131I-positive metastatic bone lesions willshow some morphological alteration. However, in thecurrent investigation, plain CT showed no organic
changes in accord with the 131I uptake. Because metastatic lesions without significant bone destruction are
missed even by 64-slice multidetector (MD)CT withexcellent image quality and high spatial resolution, theassessment of bony structures by FGD-PET CT (16slice) might also be of insufficient quality.31

Jpn J Radiol (2011) 29:413–422

417

Table 3. Possible factors predicting prognosis
FactorGood (n = 50)

Poor (n = 20)

PSex (M : F)
Age (years)
Pathology (F : P)
Differentiation (W : P)
Stage I–III : IV
Prior 131I therapy (yes : no)Thyroglobulin (ng/ml)
FDG-PET (positive : negative)
131
IWBS (positive : negative)
MR-DWI (n = 59) (positive : negative)

14 : 3658.3 ± 11.8
4 : 46
48 : 2
37 : 13
7 : 43183.5 ± 13.8
22 : 28
37 : 13
18 : 25

8 : 1268.2 ± 11.3
4 : 16
12 : 82 : 189 : 111600.8 ± 79.6
20 : 0
10 : 10
16 : 0NS
0.0041NS
0.004<0.0001
0.00530.0011<0.05
NS<0.001

F, follicular; P, papillary; W, well differentiated; P, poorly differentiated; MR, magnetic
resonance
Table 4. Factors influencing survival rate (multivariate analysis)
Variable

Relative risk

95% CIP

Age >45 years
Sex: male
Pathology: follicularThyroglobulin >30 ng/ml
Stage: IV131
I irradiation: repeated
FDG-PET: positive
DWI: positive131
I: negative

4.64
2.17
1.72
1.35
1.39
2.07
5.01
2.51
1.76

3.89–5.26
1.22–2.43
1.22–2.37
0.91–1.84
0.94–1.89
1.45–2.87
3.41–6.62
1.60–3.42
1.05–2.49

0.034
0.069
0.899
0.439
0.581
0.341
0.011
0.065
0.091

CI, confidence interval

1.0

P<0.050.8

FDG negative(n=42)
FDG positive (n=28)

P<0.05

0.8

0.6

0.6

0.4

0.4
0.2

0.2

a

DWI negative(n=34)
DWI positive (n=25)

1.0

1.5

2

2.5

3

3.5

4

4.5

b

1.5

2

2.5

3

3.5

4

4.5

Fig. 2. a Comparison of survival rates between FDG-positive and
FDG-negative groups. The survival rate for the FDG-positive
group was significantly lower than that for the FDG-negative

group. b Comparison of survival rates between DWI-positive and
DWI-negative groups. The survival rate for the DWI-positive
group was lower than that for the DWI-negative group

For the detection of lymph node metastasis, IWBS
was the most sensitive modality of the four. However,
FDG-PET/CT detected approximately 25% lesions thatwere not detected by IWBS. As papillary thyroid cancer
has the highest incidence of DTC and showed a highoccurrence of lymph node metastases,32,33 a combinationof IWBS and FDG-PET/CT is necessary to detect metastatic lesions.28,29 Although in no case was the size of thelymph nodes a problem regarding diagnosis in thecurrent study, it is difficult to evaluate the diameter oflymph nodes by low-dose plain CT alone. Contrast
enhancement CT can overcome these limitations by
accurately delineating the lymph node contour.

DWI has been proven to be useful for various thoracic diseases including pulmonary nodules such as
primary lung cancer and thyroid cancer metastasis.13–16,20,34,35 The mechanism of DWI for depicting amalignant tumor depends on the diffusing capacity of awater molecule. High-intensity-signal DWI is associatedwith cellular edema, cellular high density, fibrosis, andgelatinous and viscous fluids.17,18,34 Therefore, within
higher-grade carcinomas with high cell density, such as
poorly differentiated adenocarcinoma, the diffusing
capacity of a water molecule seems to be inhibited. Thus,
the aggressive biological behavior of various cancers canbe estimated by DWI.

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Jpn J Radiol (2011) 29:413–422

Fig. 3. A 35-year-old woman with well-differentiated papillarycancer was categorized as having a good prognosis. Whole-body
FDG-PET demonstrated no abnormal uptake (a). Chest
FDG-PET/CT showed no abnormal findings (b). DWI shows nodefinitive abnormality (c). 131I whole-body scintigraphy (131IWBS)

shows diffuse and intense uptakes in bilateral lung fields (d). The
serum thyroglobulin (Tg) level decreased from 900 ng/ml to 25 ng/
ml after the first 131I therapy. Follow-up 131IWBS after 1 year
showed no tracer uptake in the lung field (e)Although the mechanisms for visualizing aggressive
characteristics are different with the two imaging
methods, the patterns of FDG uptake and DWI highsignal intensity were in relatively good agreement andwere similar. Recently, some researchers reported that
DWI and PET showed similar sensitivity in distinguishing malignant from benign pulmonary nodules.20 They
insisted that DWI showed significantly higher specificity
than PET because it yielded fewer false-positive results
for active inflammatory lesions.20 In another comparison
study between DWI and FDG-PET/CT in regard tobreast cancer staging,36 DWI seems to be a sensitive but
unspecific modality for the detecting locoregional diseaseor metastasis. In the current study, certain discrepancies
were also observed between FDG-PET/CT and DWI
findings, in particular, lymph node metastases. One
explanation is that the hypermetabolism of glucose incancer cells is not always equal to the increase in cancer
cell density or viscosity. In some metastases with a low
density of cancer cells, hypermetabolic cancer cells canbe detected by FDG-PET/CT. In contrast, lower metabolic metastasis with a high density of cancer cells canbe identified with DWI. Another explanation could bethat there is physiological uptake with each modality.
For instance, in the thoracic region, the ribs, spinal cord,

and mediastinal lymph nodes sometimes show nonspecific increased signal intensities.35,37,38 At the same time,lung fields contain few of the protons that are necessaryfor DWI. Respiratory movement is a restriction factor
as well.39 Recently, DWI has made remarkable progresswith the development of hardware, such as the 3-teslamachine.40 The differences in findings between FDG-PET/CT and DWI may be attributable to the performance of
MRI. However, DWI could be an alternative imaging
method in the future with a sequence adjustment. Futurestudies should compare higher-performance DWI with
FDG-PET/CT findings.
With regard to the correlation between imaging patterns and prognosis, we could predict an unfavorable
prognosis in patients with FDG uptake, as several
authors previously reported.11,41–43 The survival ofpatients harboring FDG-positive tumor deposits was
significantly shorter than that of subjects with FDGnegative metastases as Robbins et al. reported.22 FDG isoften taken up by relatively poorly differentiated thyroid
cancer or dedifferentiated cancers in which the glucose
metabolism has increased5,6 and expression of glucose
transporters (GLUT-1 and GLUT-3) is increased.11
They generally grow faster than well-differentiatedthyroid cancer.44,45 Joensuu and Ahonen also reported

Fig. 4. A 60-year-old woman with differentiated thyroid cancer(DTC) (well-differentiated papillary cancer) was categorized ashaving a good prognosis. Whole-body FDG-PET demonstrated
no abnormal uptake (a). Chest FDG-PET/CT demonstrated multiple lung metastatic nodules (b). DWI shows no definitive abnor-

mality (c). 131IWBS shows multiple areas of intense uptake in thelung (d). Just after iodine therapy, the serum Tg level was 290 ng/
ml. At 6 months later, the lung uptake had decreased (e). The
serum Tg level decreased to 91 ng/ml during the follow-up
periodFig. 5. A 62-year-old man with DTC (poorly differentiated papillary cancer) was categorized as having a poor prognosis. Wholebody FDG-PET demonstrated intense uptake in the leftsubclavicular lymph node (arrow) (a), although 131IWBS shows
uptake only in the thyroid bed (b). DWI shows no definitive abnor-

mality (c). Just after the initial iodine therapy, the serum Tg levelwas 28 ng/ml. At 6 months later, multiple areas of FDG uptake
were noted in multiple organs, including the left lung, mediastinal
lymph nodes, and neck lymph nodes (d). The serum Tg level
increased to 1600 ng/ml during the follow-up period

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Jpn J Radiol (2011) 29:413–422

Fig. 6. A 75-year-old woman with DTC (well-differentiated papillary cancer) was categorized as having a poor prognosis. 131IWBS
showed no abnormal uptake (a). FDG-PET demonstrated multiple lung areas with uptake and left supraclavicular lymph node
uptake (b). DWI demonstrated a high-intensity lesion (arrow) in

the left lung (c). Chest CT showed multiple lung metastases (d). At
9 months after 131I irradiation, lung metastases had increased insize and number compared with that before therapy (e). The serum
Tg level increased from 480 ng/ml to 830 ng/ml during the followup periodthat metastases exhibiting high FDG uptake but low 131I
uptake grow rapidly.46 It has been reported in previous
studies that most PET-positive metastases are of a histological aggressive subtype.41,42 In various cancer metastasis or recurrences, higher FDG concentrations werenoted in connection with accelerated proliferative activity.47 The degree of FDG uptake may have a certain
correlation with proliferative activities, which is one of
the major factors in aggressiveness. According to somemolecular studies, FDG-positive/radioiodine-negative
metastases have a mutational profile such as RAS or
BRAF mutations.11,48,49 Such mutations in oncogenes iswell known to be present in aggressive malignancies.11
FDG positivity may be useful as a tool for estimating
the abnormality of the genes and noninvasively predicting a bad prognosis.
In general, the standard treatment for DTC includes
total surgical thyroidectomy followed by 131I therapy forresidual or metastatic disease. The follow-up is based onmonitoring the serum Tg level and IWBS. The natural
history of DTC is long, and deaths that occur ≥20 years
after the initial diagnosis can be attributed to it.30
However, if FDG-positive lesions exist at any rate during
the follow-up periods, they can potentially dedifferentiate and continuously progress despite repeated series of131
I therapy. In such cases, the therapeutic effect is insufficient, and another effective therapy, such as cytotoxic
chemotherapy, should be chosen.21 For instance, after
having given 131I therapy, an additional chemotherapy
protocol is one of the choices. To determine the optimaltreatment strategy depending on the situation, monitoring with FDG-PET/CT or DWI is necessary.
There were some limitations in the current study. The
first is the differences in the quality of the equipment.
Although FDG and IWBS showed different findings, thedifference in the findings may have been caused by thedifferent acquisition methods used for the PET/CT
camera and planar scintigraphy by the gamma camera.

Jpn J Radiol (2011) 29:413–422

A PET/CT camera has much higher sensitivity andspatial resolution than planar scintigraphy.50 Generally,
PET/CT is superior to planar scintigraphy for detecting
various metastases and providing better image quality.
Several small lesions, therefore, might be detected easily
with FDG-PET/CT but only with difficulty with IWBS
because of poor spatial resolution. In addition, 11
patients did not undergo the DWI examination, and so
the DWI data that were analyzed comprised a smaller
number. The detectability rate for DWI might improve
if more patients are analyzed.
Another limitation is the method used for the organbasis analysis. Because individual comparisons of small
and multiple lesions were difficult, solitary and multiple
metastases were regarded as one lesion in each organ. In
addition, we did not perform a quantitative analysis,
such as for the standardized uptake value or the diffusion coefficient value (ADC). Although the great overlapof ADC values existed in malignant lesions and benign
lesions including physiological uptake,36 analysis using
ADC values might be helpful when monitoring the therapeutic effect in individual cases.
Finally, the diagnosis was not always confirmed pathologically for each lesion. We performed clinical course
observations, particularly with imaging analysis, and we
could define metastases that showed 131I uptake or serial
growth. However, we did not include several smaller
lymph nodes with FDG uptake finally diagnosed as
inflammation. Nevertheless, owing to the slowly progression of DTC metastasis, we might have underestimated FDG uptake-positive and 131I-negative lesions.
Optimally, examination of biopsy would be preferable
for confirming the clinical significance of FDG-PET/CT
findings.
Conclusion
We found that FDG-PET/CT was the best modality for
detecting metastases of DTC after total thyroidectomy,
although IWBS provided complementary information.
Patients with FDG high uptake or high-intensity-signal
lesions on DWI tended to have a poor prognosis. Because
DWI provides information similar to that obtained with
FDG-PET in regard to predicting prognosis, the combination of IWBS and DWI might be the methods ofchoice for monitoring DTC.
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